ST. PETERSBURG, Russia—President Barack Obama fell short in his bid to rally international support for a U.S. military strike on Syria, failing to overcome divisions at a summit of world leaders before flying home to face skeptics in Congress and the American public.

In a news conference at the G-20 summit in St. Petersburg, Russia, President Barack Obama said that failure to respond to a chemical weapons attack in Syria would send the wrong signal to rogue nations and terrorist organizations. Obama is expected to discuss Syria on Tuesday. Photo: AP.

Mr. Obama spent two days coaxing leaders of the Group of 20 nations, pulling presidents and prime ministers aside for private conversations and making his case in a four-hour dinner at a historic palace here.

The lobbying blitz calling for a "strong international response" to Syria's alleged use of chemical weapons against rebels showed little sign of changing the minds of world leaders—some of whom took that phrase to mean "military action."

Mr. Obama's push also gained little ground in building a coalition to politically isolate Russian President Vladimir Putin, the summit host and one of the Syrian regime's chief allies and protectors.

President Obama discusses Thursday night's meeting with Russian President Vladimir Putin and other world leaders regarding their thoughts on Syria, at the G-20 Summit in St. Petersburg, Russia. Photo: AP.

The White House on Friday released a list of 11 nations including the U.S. that signed the statement, which expressed support for "efforts undertaken by the U.S. and other countries to reinforce the prohibition on the use of chemical weapons."

But Mr. Putin put out a list of his own. At a news conference on Friday, he rattled off the names of eight G-20 nations, including his own, that he said oppose international military action in Syria. Ten nations altogether declined to sign the U.S. statement.

Not all of those that did sign endorsed Mr. Obama's call for a military strike against Syria. While Italy signed, Italian Prime Minister Enrico Letta said he had told Mr. Obama that his country wouldn't join any military action in Syria.

Spain, which signed, isn't a full member of the G-20, but rather, a permanent guest—putting the vote tally among members of the group of the world's largest economies at 10-10.

Germany didn't sign—after Chancellor Angela Merkel decided to try to reach a united European position first. Secretary of State John Kerry will present the U.S. case to a meeting of EU foreign ministers over the weekend in Lithuania.

The one leader who had appeared ready to support prompt military action last month, French President François Hollande, said on Friday that he would now wait until the United Nations releases its findings about the Aug. 21 attack outside Damascus that spurred U.S. calls for an international response.

Mr. Hollande also repeated that his decision would hinge on the outcome of a vote in the U.S. Congress.

Mr. Obama was to return to the U.S. on Friday night to face the challenge of rounding up lawmakers to authorize an armed attack against the Syrian government of President Bashar al-Assad.

With Congress set to return from a recess on Monday to take up resolutions to authorize military action, it is unclear whether Mr. Obama can muster the votes needed to pass a resolution. He has won the support of a longtime foe, House Speaker John Boehner (R., Ohio), who has been urging the president to make a strong public case for military intervention.

On Tuesday, the president will deliver a televised address on Syria from the White House.

Asking a war-weary nation to support a military strike isn't easy, Mr. Obama conceded in St. Petersburg, a point that public opinion polls bear out.

"I tend to have a pretty good sense of what current popular opinion is," Mr. Obama said at the end of the summit. "And for the American people, who have been through over a decade of war now with enormous sacrifice in blood and treasure, any hint of further military entanglements in the Middle East are going to be viewed with suspicion."

Ten countries joined the U.S. in calling for a "strong international response" to a chemical-weapons attack in Syria, but half of the G-20 spurned President Obama's push for support. Colleen McCain Nelson reports on the News Hub.

He added that suspicions probably are stronger in his own party than among Republicans. But he pressed his argument that the world needs to send a message that chemical attacks can't be ignored, casting it in moral terms.

"You know, over 1,400 people were gassed. Over 400 of them were children," he said.

In a preview of the president's argument, Samantha Power, the U.S. ambassador to the United Nations, said in a speech at a Democratic-leaning think tank in Washington Friday that alternatives in Syria had been exhausted and no risk-free option remains.

"The alternative is to give a green light to outrages that will threaten our security and haunt our conscience—outrages that will eventually compel us to use force anyway down the line at far greater risk and cost to our own citizens," she said at the Center for American Progress.

The G-20: It's a Blast

World leaders gathered this week for two days of meetings of the Group of 20 in St. Petersburg, Russia. By the looks of it, they'd be happy if the summit were shorter.

Britain AFP/Getty Images

In St. Petersburg, Mr. Obama made a broader case, saying that not punishing the Assad regime would send "a signal to rogue nations, authoritarian regimes and terrorist organizations that they can develop and use weapons of mass destruction and not pay a consequence. And that's not the world that we want to live in."

Fissures over Syria were evident throughout the summit, leading to biting exchanges of rhetoric. Messrs. Putin and Obama hardly spoke on Thursday, then met privately for about 20 minutes on Friday, talking primarily about the Syrian civil war.

Mr. Putin said he would continue supplying weapons and humanitarian aid to the Syrian government in the event of a U.S. strike. He dismissed U.S. conclusions that Syrian government forces used chemical weapons, saying the gas attack was a rebel provocation.

"Will we continue to help Syria? We will," Mr. Putin said. "We're helping them now. We are supplying arms, cooperating in the economic sphere."

The U.S. has all but given up on the notion that Russia might come around and support Mr. Obama's stance, said Ben Rhodes, a top national security adviser to Mr. Obama.

Russia, he said, has "continually supported Assad no matter what the facts show and no matter what the regime does."

U.K. Prime Minister David Cameron, whose support for military action was neutralized by a vote in the British Parliament, also sounded impatient with Mr. Putin. He described himself as frustrated and called Russia an enduring obstacle in the United Nations Security Council.

"Some of the participants were saying this has to be decided by the U.N. Security Council, knowing that they themselves sat on the U.N. Security Council and could veto any decision."

One world leader who tried to bridge differences between Messrs. Obama and Putin was Ms. Merkel.

She spoke one-on-one with each of them over the course of the summit and said that, despite the tense rhetoric, both were open to a political solution—even if the U.S. were to mount a military strike.

"I felt, in my bilateral talks with the Russian president as well as with the American president, that in any case the doors aren't closed" to finding a diplomatic solution, Ms. Merkel told reporters.

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